The present invention relates to apparatus and technique for counting, measuring, and classifying liquid droplets, and is more especially directed to a technique for measuring drop size and concentration for droplets in a liquid-liquid dispersion. The invention is concerned with a laser capillary technique for near real-time measurement of bivariate drop size and concentration distribution and of monovariate drop size distribution or monovariate concentration distribution.
Contact of two immiscible liquids with one another typically involves mass transfer across a boundary between the liquids. This can occur with or without accompanying chemical reaction, and often involves liquids in stirred tanks, typical of petrochemical, hydrometallurgical, food, pharmaceutical, and polymer industries. Intimate contact is established between two liquid streams to ensure adequate rates of interphase mass transfer and or chemical reaction. This intimate contact is usually promoted by dispersing one phase as droplets in a second, continuous phase.
Many investigators have reported that the dispersed phase mixing can have a significant effect on the reactor performance. Further, theoretical work is at hand to model the effects of droplet mixing on conversion and selectivity for liquid-liquid dispersions. However, no experimental technique which can measure bivariate drop size - concentration distribution for reactive liquid-liquid dispersions has been proposed.
A variety of experimental approaches have been used to measure interfacial areas and drop size distributions in a liquid-liquid dispersions. The most common method is still the direct photography method. This method is simple, easy and accurate, but it cannot be used for high holdup fraction, especially in the case of optically dark liquid dispersion systems. The procedure usually requires many pictures and lengthy time for analysis. Direct image analysis of the data has met with only limited success for dilute dispersions.
The light transmission technique has been widely used in determining the interfacial area (or so-called Sauter mean diameter) in liquid-liquid dispersions. The method is simple and has the advantages of quick measurement and on-line operation. However, only interfacial area can be measured by this method.
The light-scattering method has been used for measurement of small drop sizes in the range below 10 .mu.m diameter, and more recently, measurement of larger drop sizes (below 800 .mu.m in diameter). Although continuous sampling is possible, the technique is limited to low dispersed phase holdup fraction (below 0.05).
The Laser Doppler Velocimetry (LDV) technique uses the doppler effect (frequency shift information) and light scattering by particles to determine drop size and drop velocity. The feasibility of the technique has been demonstrated for measurement of broad ranges of drop sizes in solid-liquid (sol) and liquid spray (aerosol) systems. However, for liquid-liquid systems there are significant limitations, especially at high dispersed phase fraction.
Drop size distribution may be obtained by using a Coulter Counter for directly observing drops suspended in an electrically conductive continuous phase. The dispersion is forced through a small aperture between two electrodes. By changing the aperture distance, a broad drop size distribution can be obtained. In this technique, however, the counting of particles necessitates the addition of undesirable conductive materials to the dispersion. The addition of these materials can affect the drop diameter and the breakup and coalescence rates in the dispersion. Furthermore, these effects are unpredictable.
The use of chemical means to measure the interfacial area has been used extensively for gas-liquid dispersions. Chemical reaction methods for determining the interfacial area of liquid-liquid dispersions requires a reactant of relatively unchanging dispersed phase concentration, which diffuses to the continuous phase. The effective interfacial area can be obtained from the extraction rate and physico-chemical properties of the system. The disadvantage of this method is the effect of the mass transfer on the physico-chemical properties of the dispersion. It has been observed that mass transfer can affect the interfacial tension and thus interfacial area.
Drop stabilization methods rely on the immediate stabilization of drops by encapsulation with thin polymer films or surfactants. A photomicrographic method has usually been employed after encapsulation of drops in the application of this technique. However, the drop stabilization method cannot be used for some liquid-liquid dispersion systems due to incompatability of encapsulating materials for those systems. This method also has disadvantages of influence of the chemical treatment on drop size. According to Verhoff et al., Breakage and Coalescence Processes in an Agitated Dispersion, Experimental System and Data Reduction, 16 Ind. Eng. Chem. Fundam. 371 (1977), a special sampling apparatus was used to withdraw a sample of dispersed phase from a mixing vessel, to stabilize drops with a surfactant, and to force the dispersed sample through a capillary with a photometer assembly to measure both size and dye concentrations of drops. This technique was applied to the system of a non-transferring, non-surface-active and non-reactive dye present only in the dispersed phase for the mixing experiments.
A capillary method employs a fine-bore capillary of the same order as the droplet sizes for sampling from the liquid dispersion in the vessel. This method can be used for obtaining broad drop size distribution in the range above 0.05 mm in diameter, at real time and automatically. One such capillary sampling system measured drop sizes in the range 0.05 to 0.6 mm for a heptane-water system in a batch reactor using a capillary of 0.2 mm in diameter. Another technique employed several different sizes of capillaries for measurement of monodispersed drops of butyl acetate in water, the drops being in the range of 2 mm to 5 mm in diameter. According to Pietzsch and Blass, A New Method For the Prediction of Liquid Pulsed Sieve-Tray Extractors, 10 Chem. Eng. Technol. 73-86 (1987), this capillary sampling method can be adapted to measure drop sizes for toluene-water and tributyl phosphate-n-alkane/water systems in a pulsed sieve-tray extraction column.
Average drop size and volume fraction of the dispersed phase in liquid-liquid systems can be measured by the acoustic wave method. The principle of that method is based on the difference between the transmission velocities of acoustic waves in the two liquids. This technique can measure, at real time, but has limitations to measure small drop sizes (below 1000 .mu.m).
The scintillation method uses short range radioactive particles for measuring interfacial area for liquid dispersion systems. However, this technique is limited by the need for high immiscibility between phases, and is further limited by the availability of suitable isotopes and target materials.
Only the technique of Verhoff et al., mentioned above, could measure bivariate size and concentration distribution of drops, although that possibility is limited to a non-reacting and non-mass transfer tracer system. The capabilities of existing capillary measurement techniques are rather limited, and do not give realistic size and concentration data in real time.